1. Field of the Invention
The present invention relates to a communication system using an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and more particularly to an apparatus and a method for transmitting/receiving pilot signals for identifying base stations and sectors.
2. Description of the Related Art
In a 4th generation (4G) communication system, which is the next generation communication system, research is currently being conducted to provide users with services having various qualities of service (‘QoS’) and that support a high transmission speed. Currently, in the 4G communication system, research is currently being conducted to support high speed services while ensuring mobility and QoS in a wireless local area network (‘LAN’) and a metropolitan area network (‘MAN’) system.
As a scheme useful for high speed data transmission in wire or wireless channels, the OFDM scheme is now actively being developed. The OFDM scheme, which transmits data using multiple carriers, is a special type of a Multiple Carrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal sub-carriers before being transmitted.
In order to provide a wireless multimedia service at high speed and high quality, the 4G communication system requires a wideband spectrum resource. However, when the wideband spectrum resource is used, not only the influence of fading on the wireless transmission paths due to multi-path propagation becomes severe, but also the frequency selective fading has an influence on the transmission frequency bands. Therefore, for high speed wireless multimedia services, the OFDM scheme is now more frequently used than the Code Division Multiple Access (CDMA) scheme in the 4G communication system, since the OFDM scheme is more robust against the frequency selective fading and is thus more advantageous than the CDMA scheme.
Operations of a transmitter and a receiver in a communication system using the OFDM scheme (OFDM communication system) will be briefly discussed. The transmitter may be a base station (BS) and the receiver may be a subscriber station (SS).
In the transmitter of the OFDM communication system, input data is modulated into sub-carrier signals by a scrambler, an encoder and an interleaver. The transmitter provides a variety of variable data rates, which determines the coding rate, the interleaving size and the modulation scheme. Usually, the encoder uses coding rates such as ½, ¾, etc., and the interleaving size for preventing burst error is determined according to the Number of Coded Bits Per OFDM Symbol (NCBPS). As the modulation scheme, a QPSK (Quadrature Phase Shift Keying) scheme, an 8PSK (Phase Shift Keying) scheme, a 16QAM (Quadrature Amplitude Modulation) scheme, or a 64QAM (Quadrature Amplitude Modulation) scheme may be used according to the data rates.
A predetermined number of the modulated sub-carrier signals are added to a predetermined number of pilot sub-carrier signals, and an Inverse Fast Fourier Transform (IFFT) unit performs IFFT for the added signals, thereby generating an OFDM symbol. Guard intervals are then inserted into the OFDM symbol in order to eliminate the inter-symbol interference (ISI) in the multi-path channel environment, and the OFDM symbol containing the guard intervals is finally input to a Radio Frequency (RF) processor through a symbol waveform generator. The RF processor processes the input signal and transmits the processed signal over the air.
The receiver of the OFDM communication system corresponding to the transmitter as described above performs a reverse process to the process in the transmitter together with an additional synchronization step. First, frequency offset estimation and symbol offset estimation are performed in advance using a training symbol set for a received OFDM symbol. Then, a data symbol obtained by eliminating the guard intervals from the OFDM symbol is restored to a predetermined number of the sub-carrier signals containing a predetermined number of pilot sub-carriers added thereto by a Fast Fourier Transform (FTT) unit. Further, in order to overcome any path delay in an actual wireless channel, an equalizer estimates the channel condition for the received channel signal, thereby eliminating the signal distortion in the actual wireless channel from the received channel signal. The data channel-estimated by the equalizer is transformed into a bit stream which then passes through a de-interleaver. Thereafter, the bit stream passes through a decoder and a descrambler for error correction and is then output as final data.
In the OFDM communication system as described above, a transmitter (for example, a BS) transmits the pilot sub-carrier signals to a receiver (for example, an SS). The BS simultaneously transmits the data sub-carrier signals together with the pilot sub-carrier signals. The SS can perform synchronization acquisition, channel estimation and BS identification by receiving the pilot sub-carrier signals. That is, the pilot sub-carrier signal is a reference sub-carrier signal and serves as a training sequence, thereby enabling channel estimation between the transmitter and the receiver. Moreover, an SS can identify, by using the pilot sub-carrier signal, a BS to which the SS belongs. The locations for the pilot sub-carrier signals, are defined in advance by a protocol between the transmitter and the receiver. As a result, the pilot sub-carrier signals operate as reference signals.
A process in which an SS identifies by using the pilot sub-carrier signals, a BS to which the SS belongs will be described.
First, the BS transmits the pilot sub-carrier signals at a transmit power level greater than that for the data sub-carrier signals such that the pilot sub-carrier signals can reach the cell boundary with a particular pattern (specifically, pilot pattern). The reason why the BS transmits the pilot sub-carrier signals with a high transmit power such that the pilot sub-carrier signals can reach the cell boundary with a particular pilot pattern will be described.
First, the SS does not have any specific information identifying the BS to which the SS currently belongs when the SS enters a cell. In order to detect the BS to which the SS belongs, the SS must receive the pilot sub-carrier signals. Therefore, the BS transmits the pilot sub-carrier signals having a particular pilot pattern with a relatively high transmit power, in order to enable the SS to detect the BS to which the SS belongs as far away as at the cell edge.
The pilot pattern is a pattern generated by the pilot sub-carrier signals transmitted by the BS. That is, the pilot pattern is generated by the slope of the pilot sub-carrier signals and the start point at which the pilot sub-carrier signals begin to be transmitted. Therefore, the OFDM communication system must be designed such that each BS in the OFDM communication system has a specific pilot pattern for its identification. Further, a coherence bandwidth and a coherence time must be taken into account when generating the pilot pattern.
The coherence bandwidth is a maximum bandwidth based on an assumption that a channel is constant in a frequency domain. The coherence time is a maximum time based on an assumption that a channel is constant in a time domain. Therefore, it can be assumed that the channel is constant within the coherence bandwidth and the coherence time. As a result, the transmission of a single pilot sub-carrier signal within the coherence bandwidth and during the coherence time is sufficient for synchronization acquisition, channel estimation and BS identification, and can maximize the transmission of the data sub-carrier signals, thereby improving the performance of the entire system. It can be said that the coherence bandwidth is a maximum frequency interval within which the pilot sub-carrier signals are transmitted, and the coherence time is a maximum time interval within which the pilot channel signals are transmitted, that is, a maximum OFDM symbol time interval.
The number of the pilot patterns having different slopes and different start points must be equal to or greater than the number of BSs included in the OFDM communication system. In order to transmit the pilot sub-carrier signals in the time-frequency domain of the OFDM communication system, the coherence bandwidth and the coherence time must be taken into consideration as described above. When the coherence bandwidth and the coherence time is taken into consideration, there is a limitation in the number of the pilot patterns having different slopes and different start points. In contrast, when the pilot pattern is generated without considering the coherence bandwidth and the coherence time, pilot sub-carrier signals in pilot patterns representing different BSs get mixed up, so that it becomes impossible to identify the BSs by using the pilot patterns.
All of the slopes which can be generated by the pilot patterns will be discussed with reference to FIG. 1.
FIG. 1 is a graph illustrating all of the slopes which can be generated by the pilot patterns in a typical OFDM communication system.
Referring to FIG. 1, all of the slopes which can be generated by the pilot patterns and the number of the slopes (that is, the slopes according to the pilot sub-carrier signal transmission and the number of the slopes) are limited by the coherence bandwidth 100 and the coherence time 110. In FIG. 1, when the coherence bandwidth 100 is 6 and the coherence time 110 is 1, if the slope of the pilot pattern is an integer, six slopes from the slop s=0 (101) to the slop s=5 (106) can be generated as the slope of the pilot pattern. That is, under the conditions described above, the slope of the pilot pattern is one integer from among 0 to 5. The fact that six slopes of the pilot patterns can be generated implies that six BSs can be identified by using the pilot patterns in the OFDM communication system satisfying the conditions described above. A hatched circle 107 in FIG. 1 represents another pilot sub-carrier signal spaced with the coherence bandwidth 100 away from the first pilot sub-carrier signal.